Rubber composition, electric wire coating material, and electric wire

ABSTRACT

A rubber composition comprising (A) a rubber ingredient comprised of (a) a nitrile group-containing highly saturated copolymer rubber having an iodine value of not larger than 120, preferably a hydrogenated product of an unsaturated nitrile-conjugated diene copolymer rubber, and (b) an acrylic ester polymer rubber, preferably a copolymer rubber comprising at least 80 mol % of alkyl acrylate or alkoxyalkyl acrylate units, at an (a)/(b) ratio of 40/60 to 90/10 by weight, and (B) an ethylene-α-olefin copolymer, the (A)/(B) ratio being in the range of 30/70 to 70/30 by weight. This rubber composition is useful as a covering material for forming a protective covering layer on electric wires.

TECHNICAL FIELD

This invention relates to a rubber composition exhibiting, whencrosslinked, good heat resistance and cold resistance, and having anespecially improved extrusion-processability. It further relates to anelectric wire having a protective covering or coating layer formed bycrosslinking the rubber composition in the state of surrounding theelectric wire.

BACKGROUND ART

Electric wire coating materials are exposed to a severe temperatureenvironment for a long term due to heat generated by application ofelectricity. Therefore they are subject to crack-formation due to heatdeterioration and sometimes insulation failure or dielectric breakdownis caused. Further, electric wire covering materials are subject toexternal heating depending upon the particular surrounding environment.

The electric wire covering materials are comprised of rubber. The rubberused includes natural rubber, a styrene-butadiene rubber and othersynthetic rubbers. When a high heat resistance is required for anelectric wire protecting covering, covering materials made of rubberssuch as a fluororubber, an ethylene-propylene-diene copolymer rubber anda silicone rubber are used. However, these rubbers have problems in thata fluororubber is expensive, an ethylene-propylene-diene copolymerrubber has a relatively low heat resistance, and a silicone rubber has apoor mechanical strength.

It is known that a blend of a nitrile group-containing highly saturatedcopolymer rubber with an acrylic rubber has a good heat resistance. As aspecific example of the rubber blend, a rubber composition comprising ablend of a hydrogenated, unsaturated nitrile-conjugated diene copolymerrubber, which is a nitrile group-containing highly saturated copolymerrubber, with an acrylic rubber, having incorporated therein an organicperoxide and a bismaleimide compound, is described in JapaneseUnexamined Patent Publication (hereinafter abbreviated to “JP-A”) No.H2-3438. This rubber composition is referred to as having good heatresistance, abrasion resistance, flexural resistance and oil resistance.The acrylic rubber contained in this rubber composition containscrosslinkable monomer units having a carbon-carbon unsaturated bond as acrosslinkable ingredient, and therefore, when a crosslinked product ofthis rubber composition is exposed to a high temperature environmenthaving a temperature of, for example, about 150° C. for long hours, itis hardened and deteriorated. Therefore, this rubber composition is notsuitable for an electric wire protective coating for which good heatresistance is required.

In conventional electric wire protective covering materials containing arubber ingredient, a resinous ingredient such as polyethylene or anethylene-vinyl acetate copolymer is often incorporated to impart a goodextrusion processability thereto. For example, when polyethylene isincorporated in an electric wire protective covering material aprotective covering made by crosslinking the protective coveringmaterial has good cold resistance, but poor heat resistance.Furthermore, when an ethylene-vinyl acetate copolymer is blended, forexample, with an elastomer having a nitrile group, 25 to 140 nitrilenitrogen atoms per 1000 carbon atoms, and not more than 35 unsaturatedbonds, as taught in JP-A S60-112839, a cable sheath having good heatresistance is obtained by radically crosslinking the blend of anethylene-vinyl acetate copolymer with an elastomer. However, an electricwire protective covering made from the ethylene-vinyl acetate copolymerwith an elastomer exhibits poor cold resistance although the protectivecovering has good heat resistance.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a rubber compositionwhich can give an electric wire protective covering having good andbalanced heat resistance and cold resistance and, even when it isexposed to a high temperature environment, the protective coating beingnot hardened nor deteriorated, and thus, keeping good rubbercharacteristics, and which rubber composition possesses good extrusionprocessability.

Another object of the present invention is to provide an electric wireprotective covering material comprising the above-mentioned rubbercomposition.

A further object of the present invention is to provide an electric wirehaving a protective covering layer having good properties, which is madeby crosslinking the above-mentioned electric wire covering material.

In one aspect of the present invention, there is provided a rubbercomposition comprising (A) a rubber ingredient comprised of (a) anitrile group-containing highly saturated copolymer rubber having aniodine value of not larger than 120 and (b) an acrylic ester polymerrubber containing at least 80% by mole of at least one kind of monomerunits selected from alkyl acrylates and alkoxyalkyl acrylate, and whichis free from a carbon-carbon unsaturated bond except for thecarbon-carbon double bonds of a benzene ring, at an (a)/(b) ratio of40/60 to 90/10 by weight, and (B) an ethylene-α-olefin copolymer, the(A)/(B) ratio being in the range of 30/70 to 70/30 by weight; saidrubber composition further comprising an organic peroxide and being freefrom an auxiliary capable of covulcanizing the rubber ingredient (a) andthe rubber ingredient (b) with the organic peroxide.

In another aspect of the present invention, there is provided anelectric wire covering material comprising the above-mentioned rubbercomposition.

In still another aspect of the present invention, there is provided anelectric wire having a protective covering layer which is formed byextruding the above-mentioned electric wire covering material around anelectric wire and crosslinking the extruded covering

BEST MODE FOR CARRYING OUT THE INVENTION

Nitrile Group-Containing Highly Saturated Copolymer Rubber

The nitrile group-containing highly saturated copolymer rubber, i.e.,ingredient (a) in the rubber ingredient (A) used in the invention, hasan iodine value of not larger than 120. The iodine value is preferablynot larger than 80, and more preferably not larger than 15. If theiodine value is too large, a crosslinked product of the rubbercomposition has poor heat resistance.

The Mooney viscosity of the nitrile group-containing highly saturatedcopolymer rubber is not particularly limited, but, in view of mechanicalstrength of a crosslinked product of the rubber composition andextrusion processability of the rubber composition, the Mooney viscosityis usually at least 15 and not larger than 200, and preferably at least30 and not larger than 100.

The nitrile group-containing highly saturated copolymer rubber can beprepared by (i) a first process of copolymerizing an unsaturated nitrilemonomer, an α-olefin monomer and an optional copolymerizable monomer, or(ii) a second process of copolymerizing an unsaturated nitrile monomer,a conjugated diene monomer and an optional copolymerizable monomer, andthen, hydrogenating the thus-obtained copolymer or terpolymer rubberwhereby at least part of the unsaturated bonds in the conjugated dienemonomer units of the copolymer or terpolymer rubber is saturated. Theα-olefin monomer used in the first process preferably has 2 to 12 carbonatoms and includes, for example, ethylene, propylene,butene-1,4-methylpentene-1, hexene-1 and octene-1. Of theabove-mentioned two processes, the second process is preferable.

As specific examples of the unsaturated nitrile monomer, there can bementioned acrylonitrile, methacrylonitrile and α-chloroacrylonitrile.These unsaturated nitrile monomers may be used either alone or incombination. Of these, acrylonitrile is preferable.

The content of nitrile group-containing monomer units in the nitrilegroup-containing highly saturated copolymer rubber is preferably atleast 10% by weight and not larger than 50% by weight, and morepreferably at least 15% by weight and not larger than 40% by weight.

As specific examples of the conjugated diene monomer to be copolymerizedwith the unsaturated nitrile monomer, there can be mentioned1,3-butadiene, 2,3-dimethylbutadiene, isoprene and 1,3-pentadiene. Theseconjugated diene monomers may be used either alone or in combination. Ofthese, 1,3-butadiene is preferable.

As specific examples of the monomer optionally copolymerized with theunsaturated nitrile monomer and the conjugated diene monomer, there canbe mentioned unsaturated carboxylic acid esters represented by acrylatesand methacrylates, which include acrylates and methacrylates having analkyl group with 1 to about 18 carbon atoms, such as methyl acrylate,ethyl acrylate, propyl acrylate, n-butyl acrylate, t-butyl acrylate,isobutyl acrylate, n-pentyl acrylate, isononyl acrylate, n-hexylacrylate, 2-methylpentyl acrylate, n-octyl acrylate, 2-ethylhexylacrylate, n-dodecyl acrylate, methyl methacrylate and ethylmethacrylate; acrylates and methacrylates having an alkoxyalkyl groupwith 2 to about 12 carbon atoms, such as methoxymethyl acrylate,methoxymethyl methacrylate, methoxyethyl acrylate, ethoxyethyl acrylate,butoxyethyl acrylate, ethoxypropyl acrylate and ethoxybutyl acrylate;acrylates and methacrylates having a cyanoalkyl group with 2 to about 12carbon atoms, such as α-cyanoethyl acrylate, β-cyanoethyl acrylate,α-cyanopropyl acrylate, β-cyanopropyl acrylate, γ-cyanopropyl acrylate,cyanobutyl acrylate, cyanobutyl methacrylate, cyanohexyl acrylate andcyanooctyl acrylate; monoalkyl unsaturated dicarboxylate and dialkylunsaturated dicarboxylates such as monoethyl maleate, dimethyl maleate,dimethyl fumarate, diethyl fumarate, di-n-butyl fumarate,di-2-ethylhexyl fumarate, dimethyl itaconate, diethyl itaconate,di-n-butyl itaconate and di-2-ethylhexyl itaconate; aminogroup-containing unsaturated carboxylates such as dimethylaminomethylacrylate, diethylaminoethyl acrylate, 3-(diethylamino)-2-hydroxypropylacrylate and 2,3-bis(difluoroamino)propyl acrylate; acrylates andmethacrylates having a fluoroalkyl group, such as trifluoroethylacrylate, tetrafluoropropyl acrylate, pentafluoropropyl acrylate,heptafluorobutyl acrylate, octafluoropentyl acrylate, nonafluoropentylacrylate, undecafluorohexyl acrylate, pentadecafluorooctyl acrylate,heptadecafluorononyl acrylate, heptadecafluorodecyl acrylate,nonadecafluorodecyl acrylate, trifluoroethyl methacrylate,tetrafluoropropyl methacrylate, octafluoropentyl methacrylate,dodecafluoroheptyl methacrylate, pentadecafluorooctyl methacrylate andhexadecafluorononyl methacrylate; fluoro-substituted benzyl acrylatesand fluoro-substituted benzyl methacrylates, such as fluorobenzylacrylate, fluorobenzyl methacrylate and difluorobenzyl methacrylate.

The optional copolymerizable monomer further includes, for example,aromatic vinyl monomers such as styrene, α-methylstyrene andvinylpyridine; non-conjugated diene monomers such as vinylnorbornene,dicyclopentadiene and 1,4-hexadiene; fluorine-containing vinyl monomerssuch as fluoroalkyl vinyl ethers, for example, fluoroethyl vinyl ether,fluoropropyl vinyl ether, trifluoromethyl vinyl ether, trifluoroethylvinyl ether, perfluoropropyl vinyl ether and perfluorohexyl vinyl ether,and o-trifluoromethylstyrene, p-trifluoromethylstyrene, vinylpentafluorobenzoate, difluoroethylene and tetrafluoro-ethylene;unsaturated monocarboxylic acids such as acrylic acid and methacrylicacid; unsaturated dicarboxylic acids such as itaconic acid, fumaric acidand maleic acid, and anhydrides thereof; copolymerizable antioxidantssuch as N-(4-anilinophenyl)-acrylamide,N-(4-anilinophenyl)methacrylamide, N-(4-anilinophenyl)cinnamamide,N-(4-anilinophenyl)-crotonamide,N-(4-anilinophenyl)amino-2-hydroxypropyl allyl ether,N-(4-anilinophenyl)amino-2-hydroxypropyl methacryl ether,5-N-(4-anilinophenyl)amino-2-hydroxypentyl acrylate,5-N-(4-anilinophenyl)amino-2-hydroxypentyl methacrylate,2-N-(4-anilinophenyl)aminoethyl acrylate,2-N-(4-anilinophenyl)aminoethyl methacrylate,N-[4-(methylanilino)-phenyl]acrylamide,N-[4-(methylanilino)phenyl]mathacrylamide, N-(4-anilinophenyl)maleimide,N-[4-(methylanilino)phenyl]-maleimide,N-phenyl-4-(3-vinylbenzyloxy)aniline andN-phenyl-4-(4-vinylbenzyloxy)aniline; and polyethylene glycol acrylate,polyethylene glycol methacrylate, polypropylene glycol acrylate,polypropylene glycol methacrylate, epoxy acrylate, epoxy methacrylate,urethane acrylate and urethane methacrylate. Of these, unsaturatedcarboxylic acid esters are preferable. Dialkyl esters of unsaturateddicarboxylic acids are especially preferable.

The monomer optionally copolymerized with the unsaturated nitrilemonomer and the conjugated diene monomer may be used either alone or incombination. The amount of the optionally copolymerized monomer isusually not larger than 50% by weight, preferably not larger than 40% byweight, based on the total weight of the monomers.

The procedure by which the nitrile group-containing copolymer rubber ishydrogenated is not particularly limited, and a conventionalhydrogenation procedure can be adopted. As examples of the catalyst usedfor hydrogenation, there can be mentioned palladium/silica, palladiumcomplexes described in, for example, JP-A H3-252405, and rhodiumcompounds and ruthenium compounds, described in, for example, JP-AS62-125858, JP-A S62-42937, JP-A H1-45402, JP-A H1-45403, JP-A H1-45404and JP-A H1-45405.

As examples of commercially available nitrile group-containing highlysaturated copolymer rubbers, there can be mentioned ZETPOL 2000, ZETPOL2020, ZETPOL 1010, ZETPOL 1020, ZETPOL 2010, ZETPOL 2000L, ZETPOL 2010Land ZETPOL 2020L, all of which are supplied by Nippon Zeon Co.

Acrylic Ester Polymer Rubber

The acrylic ester polymer rubber, i.e., ingredient (b) in the rubberingredient (A) used in the invention, is a copolymer rubber containingat least 80% by mole of alkyl acrylate units and/or alkoxyalkyl acrylateunits, and is substantially free from a carbon-carbon unsaturated bond.As preferable examples of the alkyl acrylate, there can be mentionedthose which have an alkyl group with not more than 8 carbon atoms, suchas methyl acrylate, ethyl acrylate and propyl acrylate. As preferableexamples of the alkoxyalkyl acrylate, there can be mentioned those whichhave an alkoxyalkyl group with not more than 8 carbon atoms, such asmethoxymethyl acrylate, methoxyethyl acrylate and ethoxymethyl acrylate.In view of excellent heat resistance of the crosslinked rubbercomposition, a polyacrylate rubber containing at least 80% by mole ofethyl acrylate is more preferable, a polyacrylate rubber containing atleast 90% by mole of ethyl acrylate is especially preferable, and apolyacrylate rubber containing at least 95% by mole of ethyl acrylate ismost preferable.

To produce a crosslinked rubber product having an enhanced heatresistance and exhibiting, when stored for a long term, an extremelyreduced deterioration due to curing, a monomer copolymerized with thealkyl acrylate and/or the alkoxyalkyl acrylate is free from acarbon-carbon double bond except for a carbon-carbon double bondproviding a polymerizable site and except for the carbon-carbon doublebonds of a benzene ring. The amount of the monomer to be copolymerizedwith the alkyl acrylate and/or the alkoxyalkyl acrylate is preferablynot larger than 20% by mole, more preferably not larger than 10% by moleand most preferably not larger than 5% by mole, based on the weight ofthe acrylic ester polymer rubber.

As specific examples of the monomer copolymerized, there can bementioned aromatic vinyl monomers such as styrene, α-methylstyrene andvinylpyridine; fluoroalkyl vinyl ether monomers such as fluoroethylvinyl ether, fluoropropyl vinyl ether, trifluoromethyl vinyl ether,trifluoroethyl vinyl ether, perfluoropropyl vinyl ether andperfluorohexyl vinyl ether; fluorine-containing vinyl monomers such aso-trifluoromethyl-styrene, p-trifluoromethylstyrene, vinylpentafluorobenzoate, difluoroethylene and tetrafluoroethylene; andcrosslinking monomers which include, for example, unsaturatedmonocarboxylic acids such as acrylic acid and methacrylic acid;unsaturated dicarboxylic acids such as itaconic acid, fumaric acid andmaleic acid, and anhydrides thereof; polyethylene glycol acrylate,polyethylene glycol methacrylate, polypropylene glycol acrylate,polypropylene glycol methacrylate, epoxy acrylate, epoxy methacrylate,urethane acrylate and urethane methacrylate; active chlorine-containingunsaturated monomers such as vinyl chloroacetate, allyl chloroacetate,vinylbenzyl chloride, 2-chloroethyl vinyl ether, chloromethyl vinylketone and 5-chloromethyl-2-norbornene; and epoxy group-containingunsaturated monomers such as glycidyl acrylate, glycidyl methacrylate,allyl glycidyl ether and vinyl glycidyl ether. These monomers may beused either alone or in combination.

Among the crosslinking monomers, active chlorine-containing unsaturatedmonomers are preferable. Unsaturated compounds having a carbon-carbondouble bond except for carbon-carbon double bonds of a benzene ring, inaddition to a carbon-carbon double bond providing a polymerizable site,which are conventionally used as a crosslinking agent for an acrylicrubber, are not preferable in the present invention. Such unfavorablemonomers include, for example, butadiene, 2-butenyl acrylate, ethyleneglycol dimethacrylate, triallyl isocyanurate and divinylbenzene. Ifthese crosslinking monomers are used, a crosslinked product made fromthe rubber composition becomes easily subject to hardening anddeterioration when it is exposed to a high temperature in air.Therefore, these unsaturated compounds are not used in the presentinvention.

In view of the rate of crosslinking and the rubber elasticity of acrosslinked rubber product, the amount of the above-mentionedcrosslinking monomers used is preferably in the range of 0.1 to 10% bymole, based on the total amount of the monomers for the acrylic esterpolymer rubber.

The acrylic ester polymer rubber usually has a Mooney viscosity of atleast 10 and not larger than 90, preferably at least 20 and not largerthan 70, and more preferably at least 30 and not larger than 60. If theviscosity is too small, the mechanical strength of a crosslinked rubberproduct is poor. In contrast, if the viscosity is too large, theextrusion processability is poor.

Rubber Ingredient (A)

The rubber ingredient (A) in the rubber composition of the invention iscomprised of ingredient (a), i.e., a nitrile group-containing highlysaturated copolymer rubber, and ingredient (b), i.e., an acrylic esterpolymer rubber. The ratio of ingredient (a)/ingredient (b) is in therange of 40/60 to 90/10 by weight, preferably 50/50 to 85/15 by weight,and more preferably 60/40 to 80/20 by weight.

If the amount of ingredient (a) (nitrile group-containing highlysaturated copolymer rubber) is too small, the mechanical strength of acrosslinked rubber product is poor. In contrast, if the amount ofingredient (a) is too large, the heat resistance of a crosslinked rubberproduct is not satisfactory.

The rubber composition of the invention may further have incorporatedtherein rubbers other than the rubber ingredient (A), provided that acrosslinked rubber product made therefrom has the intended heatresistance and other characteristics. As specific examples of therubbers, there can be mentioned silicone rubber, fluororubber, anethylene-acrylate copolymer rubber, an ethylene-oxide homopolymerrubber, a propylene-oxide homopolymer rubber, and a copolymer rubberpredominantly comprised of ethylene oxide and propylene oxide. Further,a liquid polymer such as an acrylonitrile-butadiene copolymer,polybutadiene or an ethylene-propylene copolymer may be incorporated inthe rubber composition. The amount of the rubber optionally used inaddition to the rubber ingredient (A) is not larger than 100 parts byweight, preferably not larger than 50 parts by weight, and morepreferably not larger than 20 parts by weight, based on 100 parts byweight of the rubber ingredient (A).

As mentioned below, it is not necessary to preliminary mix and kneadtogether ingredient (a) with ingredient (b) to prepare the rubberingredient (A).

Ethylene-α-Olefin Copolymer

The ethylene-α-olefin copolymer, i.e., ingredient (B), contained in therubber composition of the invention is a copolymer of ethylene with atleast one α-olefin, and is described in, for example, JP-A H6-306121,U.S. Pat. No. 5,194,532, ibid. U.S. Pat. No. 5,272,236 and ibid. U.S.Pat. No. 5,312,938.

The α-olefin is preferably those having 3 to 20 carbon atoms, whichinclude, for example, 1-propene, isobutylene, 1-butene, 1-hexene,4-methyl-1-pentene and 1-octene. Of these, 1-octene is especiallypreferable.

In view of the impact resistance and mechanical properties at a lowtemperature of the ethylene-α-olefin copolymer, and the balance betweenthe heat resistance and cold resistance of a crosslinked rubber product,the copolymerization ratio of ethylene/α-olefin by weight is preferablyin the range of 50/50 to 99/1, more preferably 60/40 to 97/3 and mostpreferably 70/30 to 95/5. Provided that the object and benefit of theinvention can be attained, other copolymerizable monomers may becopolymerized with ethylene and an α-olefin.

As specific examples of the copolymerizable monomer, there can bementioned styrene-type monomers such as styrene, halogenated styrene andalkyl-substituted styrene; dienes such as butadiene and 1,4-hexadienes;and naphthenes such as cyclopentene, cyclohexene and cyclooctene.

The ethylene-α-olefin copolymer usually has a substantially linearstructure, and preferably has a narrow molecular weight distribution, asexpressed by a small ratio of weight average molecular weight(Mw)/number average molecular weight (Mn), of 1.5 to 2.5. Further, thecopolymer usually has a melt index, as measured according to ASTMD-1238, of at least 0.1 and not larger than 30 g/10 min, preferably atleast 0.3 and not larger than 20 g/10 min, and more preferably at least0.5 and not larger than 10 g/10 min.

Preferable ethylene-α-olefin copolymers are commercially available andinclude, for example, ENGAGE POEs and AFFINITY POPs (bothethylene-1-octene copolymer, supplied by Dow Chem. Co.).

Rubber Composition

The rubber composition of the invention comprises the rubber ingredient(A) and ethylene-α-olefin copolymer ingredient (B) at an ingredient(A)/ingredient (B) ratio by weight of 30/70 to 70/30, preferably 35/65to 65/35 and more preferably 40/60 to 60/40. If the relative amount ofthe ethylene-α-olefin copolymer ingredient (B) is too small, theextrusion processability is poor and a stable high rate extrusion isimpossible to conduct. In contrast, if the relative amount of thecopolymer is too large, a crosslinked rubber product is rigid and haspoor rubber elasticity because of shortage of a rubber ingredient.

The rubber composition of the invention comprises an organic peroxide asa crosslinking agent for crosslinking the nitrile group-containinghighly saturated copolymer rubber (a). It is to be noted that (i) theacrylic ester polymer rubber (b) does not have a carbon-carbonunsaturated bond except for the carbon-carbon double bonds of a benzenering, and (ii) the rubber composition of the invention is free from anauxiliary capable of cocrosslinking the rubber ingredient (a) and therubber ingredient (b) with the organic peroxide, (the auxiliaryincludes, for example, bismaleimide compounds described in JP-AH2-3438). Therefore, the acrylic ester polymer rubber (b) is incapableof being crosslinked with the organic peroxide.

As specific examples of the organic peroxide incorporated as acrosslinking agent in the rubber composition of the invention, there canbe mentioned dicumyl peroxide, di-t-butyl peroxide, t-butylcumylperoxide, benzoyl peroxide,2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3,2,5-dimethyl-2,5-di(benzoylperoxy)hexyneand α,α′-bis(t-butylperoxy-m-isopropyl)benzene.

Other ingredients conventionally used in a rubber industry can beincorporated in the rubber composition of the invention according to theneed. Such ingredients include, for example, a reinforcer such as carbonblack, silica and talc, a filler such as calcium carbonate and clay, aprocessing aid, a process oil, an antioxidant, a flame-retardant, anantiozonant, a crosslinking agent, a crosslinking aid and a colorant.

The procedure by which the rubber composition of the invention isprepared is not particularly limited. The rubber composition is usuallyprepared by mixing and kneading together the nitrile group-containinghighly saturated copolymer rubber, the acrylic ester polymer rubber andthe ethylene-α-olefin copolymer, and the above-mentioned optionalingredients such as a reinforcer, a filler, a processing aid, a processoil, an antioxidant, a flame-retardant, an antiozonant, a crosslinkingagent, a crosslinking aid and a colorant.

In the preparation of the rubber composition, a procedure can beemployed wherein ingredient (A) is first prepared from ingredients (a)and (b), and then ingredient (B) is added thereto. Ingredients (a) and(b) and ingredient (B) can also be simultaneously mixed and kneadedtogether. Or, ingredient (a) and ingredient (B) are first kneaded andthen ingredient (b) is incorporated into the kneaded mixture. Theoptional ingredients may be incorporated at any stage in the course ofpreparation of the rubber composition. It also is possible toincorporate the optional ingredients in a stage of extruding the rubbercomposition for shaping.

Electric Wire Covering Material

The electric wire covering material of the invention comprises theabove-mentioned rubber composition. The electric wire covering materialis extruded into a tube form surrounding an electric wire by using aconventional extruder, and then, the tube-form extrudate is crosslinkedby applying high pressure steam or irradiation whereby a protectivecovering layer is formed around the electric wire.

The crosslinking procedure is not particularly limited, and may beappropriately chosen. When crosslinking is effected by applyinghigh-pressure steam, it is necessary to preliminarily incorporate acrosslinking agent in the rubber composition.

Electric Wire Having Protective Covering Layer

In general, an electric wire covering material in the form of a pelletis prepared by thoroughly mixing and kneading together the rubbercomposition of the invention and optional ingredients by using a closedtype mixer or an open roll and extruding the kneaded mixture into apellet. Then the covering material is extruded by an extruder at atemperature of 100 to 150° C. into a tube form surrounding a wire andhaving a thickness of 0.2 to 0.5 mm, and then, the thus-formed tube iscrosslinked.

The thus-made electric wire of the invention has a protective coveringlayer formed from the electric wire covering material. However, anitrile group-containing highly saturated copolymer rubber has a poorelectrically insulating property, and thus, to enhance the electricalinsulation of the protective covering layer, an electrically insulatinglayer is preferably formed on the outer periphery of the electric wire.The electrically insulating layer is formed from an insulating materialsuch as butyl rubber, a styrene-butadiene copolymer rubber, anethylene-propylene-diene terpolymer rubber (EPDM) or anethylene-propylene copolymer rubber (EPM).

The invention will now be specifically described by the followingexamples. The characteristics of rubber compositions and raw materialingredients were evaluated as follows.

(1) Properties of Crosslinked Rubber Product

An uncrosslinked rubber composition was prepared according to the recipeshown in Table 1, and then crosslinked at a temperature of 170° C. for20 minutes to prepare a sheet having a thickness of 2 mm. According JISK6251, #3 dumbbell specimens were cut from the sheet, and their tensilestrength (unit: MPa), 100% modulus (unit: MPa) and elongation (unit: %)were measured. Hardness was measured according to JIS K6253 by using anA-type durometer.

(2) Heat Aging in Air

A specimen was allowed to stand at 150° C. in air for 168 hours or 508hours. Then an elongation was measured, and a change in % of theelongation based on the elongation (%) as measured before thehigh-temperature exposure was determined. The minus value for theelongation change means that the elongation was lowered by thehigh-temperature exposure.

(3) Gehman Tortion Test

An uncrosslinked rubber composition was prepared according to the recipeshown in Table 1, and then crosslinked at a temperature of 170° C. for20 minutes to prepare a sheet having a thickness of 2 mm. According JISK6261, specimens were prepared, and their cold resistance was evaluated.The cold resistance was expressed by an index of T100 (unit: ° C.). Thelower the index value, the better the cold resistance.

(4) Extrusion Test

An uncrosslinked rubber composition prepared according to the recipeshown in Table 1 was extruded through a screw-type extruder having acylinder temperature of 60° C. and a head temperature of 80° C. with adie satisfying the stipulations of ASTM D2230, method A, whereby theextrusion processability was evaluated. The evaluation was conducted onextrusion rate (unit: g/min), extrusion length (unit: cm/min), and shapeand appearance of the extrudate, i.e., (1) occurrence of swell andfoams, (2) continuity and sharpness of an edge with angle of 30°, (3)surface smoothness and (4) continuity and sharpness of an edge withangle other than 30°, according to the method A. Ratings from 1 to 4were assigned for the shape and appearance. The larger the totalratings, the better the extrusion processability.

EXAMPLES 1-4 Comparative Examples 1-4

Using a nitrile group-containing highly saturated copolymer rubber, anacrylic ester copolymer rubber, an ethylene-vinyl acetate copolymer andethylene-1-octene copolymers, and other ingredients shown in Table 1,crosslinkable rubber compositions were prepared by mixing and kneadingthese polymers and ingredients by using a 1.7 liter Banbury mixer and anopen roll. The properties of crosslinked rubber products, elongationchange after exposure to high temperature conditions, Gehman tortionaltemperature, and Garvey die extrusion processability were evaluated, andthe results are shown in Table 2.

TABLE 1 Comp. Examples Examples 1 2 3 4 1 2 3 4 Recipe HNBR *1 100 70 4228 42 42 28 28 AR1 *2 — 30 18 12 18 18 12 12 EVA *3 — — 40 60 — — — —Ethylene-1-octene copolymer (1) *4 — — — — 40 60 — — Ethylene-1-octenecopolymer (2) *5 — — — — — — 40 60 Antimony trioxide 10 10 10 10 10 1010 10 Hydilight H-42 *6 60 60 60 60 60 60 60 60 Carplex 1120 *7 20 20 2020 20 20 20 20 Silane A-172 *8 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5Diethylene glycol 2 2 2 2 2 2 2 2 Nocrac 224 *9 1.5 1.5 1.5 1.5 1.5 1.51.5 1.5 Nocrac MBZ *10 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 TMPT *11 3 3 3 33 3 3 3 Percumyl D-40 *12 8 8 8 8 8 8 8 8 Note: *1 HNBR: Hydrogenatedproduct of acrylonitrile-butadiene copolymer (bound acrylonitrilecontent: 36 weight %, iodine value: 4, Mooney viscosity: 65) *2 AR1:Ethyl acrylate (95 weight %)-vinyl chloroacetate (2 weight %) copolymerrubber (Mooney viscosity: 50) *3 EVA: Ethylene-vinyl acrylate copolymer(vinyl acetate content: 26 weight %) *4 Ethylene-1-octene copolymerrubber (1) “ENGAGE 8100” (melt index: 1.0 g/10 min, supplied by DowChem. Co.) *5 Ethylene-1-octene copolymer rubber (2) “ENGAGE 8200” (meltindex: 5.0 g/10 min. supplied by Dow Chem. Co.) *6 Hydilight H-42:Aluminum hydroxide (supplied by Showa Denko K.K.) *7 Carplex 1120:Silica (supplied by Shionogi & Co.) *8 Silane A-172:Vinyl-tris-β-methoxyethoxysilane (supplied by Nippon Unicar Co.) *9Nocrac 224: 2,2,4-trimethyl-1,2-dihydroquinoline (supplied by OouchiShinko K.K.) *10 Nocrac MBZ: 2-mercaptobenzoimidazole zinc salt(supplied by Oouchi Shinko K.K.) *11 TMPT: Trimethylolpropanetrimethacrylate *12 Percumyl D-40: Dicumyl peroxide (supplied by NOFCorp.)

TABLE 2 Comp. Examples Examples 1 2 3 4 1 2 3 4 Physical properties ofcrosslinked product Tensile strength (MPa) 21.2 11.2 12.2 13.1 13.0 14.611.3 12.6 Elongation (%) 450 610 400 280 470 440 400 360 100% Modulus(MPa) 4.3 1.6 3.2 5.6 3.6 4.8 4.5 5.8 Hardness (Duro-A) 72 69 74 78 7680 78 81 Heat aging in air Elongation change (%) 168 hours' aging (%)−26 −24 −22 −18 −20 −18 −22 −19 504 hours' aging (%) −45 −23 −24 −20 −21−20 −23 −20 Gehman tortional test T100 (° C.) −46.2 −29.2 −28.2 −27.6−46.3 −47.2 −45.0 −46.5 Garvey die extrusion test Extrusion rate (g/min)32 39 42 44 45 46 42 44 Extrusion length (cm/min) 83 76 89 86 85 88 8285 Evaluation of shape and appearance Swell, foam *1 2 3 4 4 4 4 4 4Surface smoothness 1 3 4 4 4 4 4 4 Corners *2 1 3 4 4 4 4 4 4 Edges *4 13 4 4 4 4 4 4 Total ratings 5 12 16 16 16 16 16 16 Note: *1 Occurrenceof swell and foams *2 Continuity and sharpness of an edge with angleother than 30° *3 Continuity and sharpness of an edge with angle of 30°

As seen from Table 2, a crosslinked nitrile group-containing highlysaturated copolymer rubber exhibits good cold resistance, but the changeof elongation as measured after heat aging in air is large, and the heatresistance is poor. Further, the extrusion processability is poor, andthe shape and appearance of extrudate are not satisfactory (ComparativeExample 1).

A crosslinked rubber product of a blend of a nitrile group-containinghighly saturated copolymer rubber and an acrylic ester copolymer rubberexhibits good heat aging resistance, but the cold resistance is −29° C.,i.e., unsatisfactory (Comparative Example 2). The extrusioncharacteristics are acceptable, but, an increased extrusion rate issometimes difficult to adopt, although it is desired for a recentlyprogressing extrusion technique.

A crosslinked rubber product of a blend of a nitrile group-containinghighly saturated copolymer rubber, an acrylic ester copolymer rubber andan ethylene-vinyl acetate copolymer exhibits good heat aging resistanceand extrusion processability, but the cold resistance is −28° C., i.e.,unacceptable and does not satisfy the requirements of the recentlyprogressing technique (Comparative Examples 3and 4).

In contrast, as seen from Examples 1 to 4, a crosslinked product of therubber composition of the invention exhibits good heat resistance and,even when it is exposed to a high temperature environment for a longterm, it is not hardened nor deteriorated and keeps good rubbercharacteristics. The shape and appearance of extrudate is evaluated astotal rating of 16 and the extrusion processability is satisfactory.Further, the Gehman tortional temperature T100 is lower than −45° C.,i.e., the cold resistance is excellent.

INDUSTRIAL APPLICABILITY

The rubber composition of the invention exhibits, when crosslinked, goodand balanced heat resistance and cold resistance, and has especiallyhigh extrusion processability.

This rubber composition can be used as a covering or coating materialfor electric wires for an automobile use and an industrial use, and as araw material for an oil hose used in an automobile industry. The rubbercomposition is especially useful as a protective covering material forelectric wires which are exposed to a high temperature environment for along term.

The electric wire covering material comprised of this rubber compositionhas good extrusion processability and exhibits, when crosslinked,excellent heat resistance and cold resistance. Therefore, the electricwire coating or covering material is especially suitable for a coveringused for the purpose of bundling, insulating protection, heat-resistantprotection or mechanical protection of a group of mutually insulatedconductors.

The electric wire having a protective covering made from the electricwire covering material of the invention is characterized in that theprotective covering has excellent heat resistance and cold resistance,and therefore, the electric wire withstands a severe change of thesurrounding environment, and only a single kind of electric wire can beused in various environments.

What is claimed is:
 1. A rubber composition comprising (A) a rubberingredient comprised of (a) a nitrile group-containing highly saturatedcopolymer rubber having an iodine value of not larger than 120 and (b)an acrylic ester polymer rubber containing at least 80% by mole of atleast one kind of monomer units selected from alkyl acrylates andalkoxyalkyl acrylate, and which is free from a carbon-carbon unsaturatedbond except for the carbon-carbon double bonds of a benzene ring, at an(a)/(b) ratio of 40/60 to 90/10 by weight, and (B) an ethylene-1-octenecopolymer, the (A)/(B) ratio being in the range of 30/70 to 70/30 byweight; said rubber composition further comprising an organic peroxideand being free from an auxiliary capable of covulcanizing the rubberingredient (a) and the rubber ingredient (b) with the organic peroxide.2. A rubber composition according to claim 1, wherein the nitrilegroup-containing highly saturated copolymer rubber has an iodine valueof not larger than
 80. 3. A rubber composition according to claim 1,wherein the nitrile group-containing highly saturated copolymer rubberhas an iodine value of not larger than
 15. 4. A rubber compositionaccording to claim 1, wherein the nitrile group-containing highlysaturated copolymer rubber has a Mooney viscosity of 15 to
 200. 5. Arubber composition according to claim 1 wherein the nitrilegroup-containing highly saturated copolymer rubber has a Mooneyviscosity of 30 to
 100. 6. A rubber composition according to claim 1wherein the nitrile group-containing highly saturated copolymer rubbercontains 10 to 50% by weight of unsaturated nitrile units.
 7. A rubbercomposition according to claim 1, wherein the nitrile group-containinghighly saturated copolymer rubber contains 15 to 40% by weight ofunsaturated nitrile units.
 8. A rubber composition according to claim 1,wherein the alkyl acrylates have an alkyl group with not more than 8carbon atoms, and the alkoxyalkyl acrylates have an alkoxyalkyl groupwith not more than 8 carbon atoms.
 9. A rubber composition according toclaim 8, wherein the acrylic ester polymer rubber is a copolymer rubbercontaining at least 80% by mole of ethyl acrylate units.
 10. A rubbercomposition according to claim 8, wherein the acrylic ester polymerrubber is a copolymer rubber having copolymerized therein 0.1 to 10% byweight, based on the weight of the rubber, of crosslinking monomerunits.
 11. A rubber composition according to claim 8, wherein theacrylic ester polymer rubber has a Mooney viscosity of 10 to
 90. 12. Arubber composition according to claim 8, wherein the acrylic esterpolymer rubber has a Mooney viscosity of 20 to
 70. 13. A rubbercomposition according to claim 1, wherein the (a)/(b) ratio is in therange of 50/50 to 85/15 by weight.
 14. A rubber composition according toclaim 1, wherein the (a)/(b) ratio is in the range of 60/40 to 80/20 byweight.
 15. A rubber composition according to claim 1, wherein thecopolymerization ratio of ethylene to 1-octene in the ethylene-1-octenecopolymer is in the range of 50/50 to 99/1 by weight.
 16. A rubbercomposition according to claim 1, wherein the copolymerization ratio ofethylene to 1-octene in the ethylene-1-octene copolymer is in the rangeof 60/40 to 97/3 by weight.
 17. A rubber composition according to claim1, wherein the ethylene-1-octene copolymer has a melt index of 0.1 to30.
 18. A rubber composition according to claim 1, wherein theethylene-1-octene copolymer has a melt index of 0.3 to
 20. 19. A rubbercomposition according to claim 1, wherein the (A)/(B) ratio is in therange of 35/65 to 65/35 by weight.
 20. A rubber composition according toclaim 1, wherein the (A)/(B) ratio is in the range of 40/60 to 60/40 byweight.